Treatment of articles having metallic core with boron coating

ABSTRACT

THE MECHANICAL PROPERTIES, PARTICULARLY THE TENSILE STRENGTH, OR ARTICLES COMPRISING A LAYER OF BORON ON A METALLIC, PARTICULARLY TUNGSTEN, CORE ARE SUBSTANTIALLY INPROVED BY A TREATMENT COMPRISING IMMERSION IN AN AQUEOUS ACID BATH CONTAINING HYDROGEN PEROXIDE AND APPLICATION OF A POTENTIAL DIFFERENCE BETWEEN A METALLIC CATHODE AND THE ARTICLE AS ANODE.

"United States Patent ()fice 3,694,333 Patented Sept. 26, 1972 76,867 Int. Cl. C23b 1/00, 3/06 US. Cl. 204-140 5 Claims ABSTRACT OF THE DISCLOSURE The mechanical properties, particularly the tensile strength, of articles comprising a layer of boron on a metallic, particularly tungsten, core are substantially improved by a treatment comprising immersion in an aque ous acid bath containing hydrogen peroxide and application of a potential diflerence between a metallic cathode and the article as anode.

This application is a continuation-in-part of US. patent application 881,875 which was filed Dec. 3, 1969 and now abandoned.

The present invention is concerned with a process for the treatment of articles having an adherent layer of boron on a metallic, particularly a tungsten, core.

It is known to prepare articles, more particularly elongated articles, such as ribbons, filaments or fibres, by the deposition of a layer of boron on a suitably heated metallic substrate. Such articles are typically prepared by effecting the decomposition of a gaseous mixture consisting of a boron compound, such as boron tn'chloride, and hydrogen, on the substrate.

The present invention is concerned with a process for improving the mechanical properties of such articles, this process comprising immersing the articles in an aqueous acidic bath containing hydrogen peroxide and establishing a potential difference between a metallic cathode, preferably of tubular form, and the article as anode.

The passage of the articles treated in the aqueous acid bath, as defined above, produces the elimination of a more or less thick superficial layer of boron; the improvement of the mechanical properties of the objects treated appears to be associated with this elimination, as this superficial layer comprises cracks which form beginnings of rupture for the whole material. Of course, the thickness of the superficial layer to be removed according to the invention depends upon the thickness of the coating of boron of the material treated; it has been found that the thickness of the removed layer must be of an order of 1 to 5 microns Without, however, ever exceeding one third of the total thickness of the boron layer.

Moreover, it is desirable to complete the process according to the invention by subjecting the articles previously treated in the aqueous acid bath to another bath containing at least one alkaline ferricyanide. The temperature of this second bath must be between 45 and 90 C. and the duration of immersion in this bath must be between and 300 seconds. As alkaline ferricyanide, potassium ferricyanide is preferably used at a concentration of an order of 10%; the pH of this bath must be alkaline, therefore an alkaline base is generally added thereto; a ferricyanide bath is preferably used having a pH higher than 12. As there seems to be interaction between the boron of the article treated and the solution of ferricyanide used, it is desirable to agitate said solution; this agitation may be effected by any known means, such as for example by bubbling compressed air or closed circuit circulation of said solution.

Of course, as in the known processes of treating various articles in aqueous acid or basic baths, it may be desirable for said articles to be washed previously or between the two treatments mentioned above, by passage in one or more baths containing for example water, alcohol, acetone, trichlorethylene, etc.

The process according to the invention may be used by all materials having a metal core coated by an adherent deposit of boron; however, the preferred materials used in the process acording to the invention are filaments or fibres having a tungsten core and a deposit of boron which is at least sufficiently thick for the whole of this tungsten core to be converted into a tungsten boride having good mechanical properties. The process may be carried out continuously or discontinuously.

The following non-limiting examples illustrate the invention.

EXAMPLE 1 A tungsten filament coated with boron is used; the total diameter of the filament is 94 the tungsten core on which the boron layer has been deposited had a diameter of 12.5;4. This filament is washed by passage into a bath of trichlorethylene, then in a bath of alcohol. It then passes into a bath which contains, per 500 g. of water, 20 to 60 cc. of 110 volume hydrogen peroxide and a small quantity, for example 1 cc., of an acid electrolyte, such as sulphuric acid or nitric acid. There is established between the filament forming anode and the metal recipient containing the bath, a potential difference of 10 v., the current density on the filament being of an order of l amps/sq.dm. The time spent by the filament in this bath is of the order of seconds. The filament then passes into at least one finishing bath containing methyl alcohol.

The diameter of the filament after treatment is reduced to 92p; the breaking strength of the filament before treatment was 377 hbars; that of the filament after treatment is 409 hbars. The same process was carried out on filaments of tungsten coated with boron whose diameter was 9.6, 97 and 102 1. and whose breaking strength (in hbars) was 372-368 and 339 respectively. After the treatment, carried out in such a manner that thicknesses of boron, equal to 6, 1 and 2, respectively, were removed from the said filaments, filaments were obtained having breaking strengths of 425-504 and 350 hbars respectively.

EXAMPLE 2 An untreated filament with a tungsten core coated with boron passes into a bath containing a mixture of 75 parts of methyl alcohol and 25 parts of acetone; it then passes through a treatment tank according to the main patent which comprises, per 500 cm. of water, from 20 to 60 cm. of volume hydrogen peroxide and a very small quantity of an acid, the bath obtained having a pH of about 4; the potential difference the filament and the cathode surrounding said filament is established between 5 and 10 v. and the current density is of the order of 1 amps/sq.dm.; the time spent by the filament in this first bath is about 75 seconds. The filament then passes into a bath of distilled water, then into a bath containing a mixture of 75 parts of methyl alcohol and 25 parts of acetone.

The filament is then sent to a bath containing 10%- potassium ferricyanide whose pI-I has been taken to 14 by sodium hydroxide. The temperature of the bath is fixed at 80 C. and the time spent by the filament in this bath is 40 seconds.

The filament is then washed and tested.

It has been found that the second treatment further permitted a slight increase in the mechanical properties of the filament; however, the essential advantage of this treatment is to regularise the properties of the filament treated for the whole of a given batch of filament and thus to increase the reliability of this filament for its later uses.

What is claimed is:

1. A process for improving the mechanical properties of articles made by depositing a layer of boron on a metal core, more particularly made of tungsten, which comprises the steps of immersing said articles in an aqueous acid bath, containing hydrogen peroxide, and establishing a potential difierence between a metal cathode and the immersed article which forms anode, so as to eliminate a superficial layer of boron of thickness at least equal to 1 and at the most equal to one third of the thickness of the boron layer.

2. A process as defined in claim 1, wherein the density of the electric current at the surface of the treated piece is between 0.5 and 5 amps/sq.dm., the potential difference between the anode and the cathode is between 5 and v. and the duration of treatment is between 1 and 10 minutes.

3. A process of treating articles with metal core coated with boron, said articles having been treated in an aqueous acid bath containing hydrogen peroxide and in which they were subjected to a potential difierence between a metal cathode and themselves forming anode, as defined in claim 1, and wherein in addition said articles are later immersed for a period of time of between 10 and 300 seconds in a basic bath containing at least one alkaline ferricyanide and the temperature of which is between and C.

4. A process as defined in claim 3, wherein the basic bath has a pH higher than 12.

5. A process as defined in claim 3, wherein the basic bath contains 10% by weight of potassium ferricyanide.

References Cited UNITED STATES PATENTS 1,663,564 3/1928 Rich 204 1,731,269 10/1929 Rich 204-140 2,382,549 '8/ 1945 Edmonson 204-140 GERALD L. KAPLAN, Primary Examiner T. 'IUFARIELLO, Assistant Examiner US. Cl. X.R. 204140.5, 141 

